The invention relates to a flame ionization detector (FID) which comprises a supply and an ignition device for the combustible gas, a supply for the sample gas, a combustion chamber in which the sample gas is ionized by the flame, and electrodes to which a voltage is applied in order to generate and measure the ion current.
Flame ionization detectors are used to detect and measure volatile organic compounds in gaseous samples. The measurement is based on the chemical ionization of organic substances, which are pyrolyzed in an oxyhydrogen gas flame. An ionization reaction of the carbon atoms contained in the substance takes place:CH+O→CHO++e−
When a voltage is applied to an electrode pair arranged at the edge of the flame, an ion current flows which can be measured and used to detect the organic compounds. If the gas is first passed through a gas chromatograph, for example a capillary gas chromatograph, then the various chemical compounds of the sample gas enter the flame ionization detector in succession, sorted according to molecular weight, so that the concentration of the different components can be established.
A problem with flame ionization detectors is that it is necessary to supply oxyhydrogen gas, a highly explosive mixture of oxygen and hydrogen. It is therefore desirable to make the flame ionization detectors as small as possible so that only small quantities of oxyhydrogen gas are required, and the explosion risk is therefore reduced. Furthermore, such small flame ionization detectors are naturally advantageous since they are easier to transport and take up less space. In addition, the lower consumption of oxyhydrogen gas allows it to be produced in situ by electrolysis instead of being used in a stored form, which further reduces the explosion risk. One such flame ionization detector, which makes use of this advantage, consists of components that are produced according to the methods of microsystem technology (S. Zimmermann et al., “Microflame ionization detector and microflame spectrometer”, Sensors and Actuators B63 (2000), pp. 159-166; S. Zimmermann et al., “Miniaturized flame ionization detector for gas chromatography”, Sensors and Actuators B83 (2000), pp. 285-289). The oxyhydrogen flame in this case burns in open space, and is enclosed only by a metallized glass tube which forms an electrode pair together with the silicon substrate. Since the flame burns in open space, the result can be influenced by turbulence and contamination. Heat is furthermore radiated, so that a comparatively large quantity of combustible gas is required. An additional disadvantage is that the glass tube must be adhesively bonded, and therefore the detector cannot be produced entirely by the methods of microsystem technology, so that its structure is elaborate and expensive and unsuitable for mass production.
Other previously known flame ionization detectors with a small design have the disadvantage that they cannot be produced, or cannot be produced entirely, by the methods of microsystem technology (U.S. Pat. No. 5,576,626; WO 2006/000099 A1).